Hexane-1,6-diammonium pentaiodobismuth (HDA-BiI₅) is one of the narrowest bandgap molecular semiconductor reported in recent years. Through the study of its energy band structure, it can be identified as an N-type semiconductor and is able to absorb most of the visible light, making it suitable to fabricate solar cells. In this paper, SnO₂ was used as an electron transport layer in HDA-BiI₅-based solar cells, for its higher carrier mobility compared with TiO₂, which is the electron transport layer used in previous researches. In addition, the dilution ratio of SnO₂ solution has an effect on both the morphology and photophysical properties of HDA-BiI₅ films. At the dilution ratio of SnO₂:H₂O = 3:8, the HDA-BiI₅ film has a better morphology and is less defect inside, and the corresponding device exhibited the best photovoltaic performance.

Perovskite light-emitting diodes (PeLEDs) have attracted tremendous attention due to their ideal optoelectronic properties, such as high color purity, high fluorescence quantum yield, and tunable light color. The perovskite layer plays a decisive role in the performance of PeLEDs and the solvent engineering of the perovskite layer is the key technological breakthrough in preparing high quality films. In this study, we have proposed the strategy of adding different amounts of solvents to the perovskite precursor solution to optimize the morphology of perovskite films and device performance. As a result, with the decreasing concentration of perovskite precursor solution, the perovskite film morphology is smoother and more favorable for carrier injection and combing, which induces an enhanced external quantum efficiency. The maximum luminance of PeLEDs was increased from 1667 cd/m² to 9857 cd/m² and the maximum current efficiency was increased from 6.7 cd/A to 19 cd/A. This work provides a trend to achieve improved film morphology and device performance for perovskite optoelectronic devices.